Suspension systems for reciprocating mechanical devices have been widely known and used in many different fields of endeavor. Typically their role is to constrain reciprocating movement to a straight line over a designated stroke length in various devices such as piston pumps, fuel pumps, solenoids and other actuators, linear motors and the like. An inherent primary design problem is how to accommodate the geometric need for a diaphragm or end-fixed flexure strips to stretch to greater length at the stroke-ends.
In the present field of endeavor, electromagnetic actuators, oscillating drivers and the like, the evolution that has taken place in the still ongoing search for satisfactory suspensions has included many different approaches including sliding bearings, coil springs and non-metallic resilient materials, each of which have been found less than ideal in this field of endeavor. Flat spring flexures offer high ruggedness and reliability but cannot be directly attached at both ends if substantial stroke length is required.
Known actuators have been configured with the mass traversed by and secured to a concentric shaft supported by a pair of base-affixed bearings flanking the mass, the mass and shaft being driven to reciprocate longitudinally along the central axis of the shaft.
Examples of the bearing method of suspension of a movable member mounted concentrically on an axially moving shaft are found in U.S. Pat. No. 4,924,123 to T. Hamajima et al for a LINEAR GENERATOR wherein the mass includes permanent magnets, and in U.S. Pat. No. 5,231,336 to F. T. van Namen for ACTUATOR FOR ACTIVE VIBRATION CONTROL wherein the moving mass can be magnet(s) or coil(s).
In another form of bearing suspension the reciprocating mass may be formed as a piston whose outside surface slides against the surrounding cylindrical bore of the stator portion, as exemplified by U.S. Pat. No. 4,454,426 to G. M. Benson for a LINEAR ELECTROMAGNETIC MACHINE.
The use of bearings has the disadvantages that they introduce friction and loss of efficiency and are subject to wear deterioration over time, even when kept well lubricated. These disadvantages accelerate and compound rapidly toward failure if lubrication becomes depleted.
As a further disadvantage, bearings alone do not provide any restoring force e.g. the '123 Hamajima patent shows a shaft 1 free to move axially in bearings 2 with no flexures or other axial constraint within the stroke length. Thus bearing-type suspensions are often augmented by compliant suspension elements such as coil springs in some form, e.g. an opposed pair, one at each end, in order to ensure desired positioning of the armature shaft within the stroke length, and to also create a mechanical resonance at a frequency determined by mass and compliance, i.e. spring constant.
The disadvantages of the bearings/coil springs approach can be eliminated and their respective functions can both be performed simultaneously and advantageously by the alternative approach of deploying a flexure type suspension system.
A basic flexure component could made inexpensively in the form of a resilient flat diaphragm as in a drumhead, however if fastened around the perimeter, the material must be fully resilient, i.e. stretchable, in all directions, to allow for radial variations due to geometric variations in the span throughout the stroke.
Resilient stretchable non-metallic materials are generally not sufficiently durable, stable and reliable enough to meet certain stringent demands that can be met only with flexures of solid material such as flat springs.
However flexure elements often need to be retained at one end by some form of slippage or resilient mounting,
U.S. Pat. No. 5,896,076 to Frederik T. Van Namen, assigned to Motran Industries, discloses a FORCE ACTUATOR WITH DUAL MAGNETIC OPERATION for active vibration purposes, utilizing a combination of both voice coil and solenoid electro-magnetic principles in conjunction with a suspension system combining (1) a two-tier flexure system having a pair of flexure assemblies flanking the armature mass, each assembly having four radial flexure span elements in a quad polar pattern, the outer end of each span element being supported by resilient non-metallic material and (2) a pair of opposing coil springs under compression, flanking the armature mass.
Compliant materials are widely utilized, e.g. in the corrugated or foam surround cone suspension, or spiral flexures as in U.S. Pat. No. 6,850,138 to N. Sakai for a VIBRATION ACTUATOR HAVING AN ELASTIC MEMBER BETWEEN A SUSPENSION PLATE AND A MAGNETIC CIRCUIT DEVICE. Such spiral structure imparts undesirable rotation about the axis of the “spider” for voice coil suspension of consumer products such as loudspeakers where the requirements are less stringent than, for example, aerospace, where the demands generally dictate stiffer flexure systems.
In FIGS. 1A, 1B, and 1C are shown three views of an inertial actuator developed by Motran Industries for active vibration control in aircraft, utilizing the voice-coil principle in conjunction with an all-metal flexure system utilizing stacked horizontal flat spring elements supported at outer ends by relatively stiff vertical flexure elements.
In FIG. 1A, the stator portion affixed to base 12 includes a pair of tubular bobbins 10A of which the bottom ends are seen located beneath associated shell elements 10 of the vibratable armature mass portion wherein bobbins 10A carry annular voice coils. The vibratable armature mass includes central permanent magnets stacked with circular pole-pieces surrounded by the ferrous metal tubular shell elements 10, forming an annular magnetically-charged gap traversing each coil on bobbins 10A. A yoke-piece 10B links the cores mechanically and magnetically to the shell, and an auxiliary mass 14 connects the vibratable armature mass portion to a vibratable central region of the flexure assembly constituting the moving armature node.
The flexure assembly consists of a horizontal stack 16 of spring flexure strips supporting the armature mass at the center, and extending in mirror image as two opposed span elements to the two ends where each is supported by a vertical stack of rigidly attached flat springs 18 whose lower end is firmly attached to a corresponding end of the base 12.
Being co-linear, the two flexure span elements can be integrated, i.e. formed as a stack of flat spring span elements extending full length as shown in the plan view, FIG. 1B. Each span element is contoured in width as shown, approximating the shape of a double hourglass, each hourglass being located above a corresponding shell element 10.
The vertical secondary span support elements 18 are typically made to be much stiffer than the horizontal primary span elements 16 that they support, and are designed for minimal influence affecting the basic performance of the overhead flexure stack.
FIG. 2A shows, in simplified form, the basic flexure system of the actuator of FIG. 1 which includes base 12, armature mass 14A and the horizontal flexure stack 16 which forms a pair of in-line flexure spans, each having its free end attached rigidly at perpendicular corner to a relatively stiff vertical flexure support 18. In the unpowered quiescent condition shown, the armature mass 14A is located at its neutral mid-stroke location; the two flexure span elements 16 and two supports 18 are seen to be undeflected and linear.
FIGS. 2B and 2C show a left hand portion of FIG. 2A with the armature mass 14A having been driven to the top and bottom of its stroke respectively. In both cases there is a similar horizontal displacement at the upper end of vertical support 18, relative to the undeflected location shown in broken lines, due to the effective shortening of the main flexure span element 16 when fully deflected. The rigid corner results in the S-shape of the flexure span element 16 under stress of deflection, also seen to a lesser degree in the stiffer and longer support 18.
The horizontal displacement of support 18 represents vibration in the two corner regions at twice the rate of the mass portion 14A and having a non-sinusoidal waveform rich in harmonic distortion. To the extent that the direction of this spurious vibration could be made exactly perpendicular to the stroke axis, perfect left/right symmetry of the flexure structure would totally cancel the effect of the spurious vibration on the mass, so that the stroke would be unaffected. However, with a long stroke, due to geometric non-symmetry in the vertical direction inherent in this configuration of flexure supports 18, some off-axis components that fail to cancel may act upon the mass portion 14A, introducing harmonic distortion in the stroke drive force which could result in unwanted audible noise and/or loss of efficiency.
The flexure supports 18 need to be optimized to mitigate unwanted influence of their side-effects on the desired performance of the main flexure span elements 16.
In the design of actuators, higher output force requires a longer actuator to accommodate a longer stroke, or an actuator with a large diameter to accommodate additional inertial mass. A lower operating frequency also requires a longer stroke, which requires very long coil springs or flexures, adding to the bulk of the actuator and reducing the ratio of active mass to total mass. The increased dimensions and mass of such actuators tend to make them unsuitable for some special deployments, e.g. for helicopters.
It is a primary object of the present invention to provide superior structure for linear electromagnetic actuators, particularly with regard to mass-suspension, that are improved over those of known art regarding overall operational electrical efficiency, reliability, uniformity of stroke, cost-effectiveness and ease of production.
It is a further object to provide such improvements, while still utilizing known and proven magnet and coil bobbin components.
It is a further object to provide an improved flexure system in both rectangular and radial arrangements that operate under lower and more uniformly distributed stress, applicable to a wide range of applications including active vibration generation and suppression.
It is a further object to provide a system wherein all flexure stacks are all uniform in length.
It is a further object to arrange for each flexure member to be made from two functional span elements of equal compliance.
It is a further object to provide a family of flexure systems utilizing uniform flat springs as basic building blocks, that can be stacked to form flexure span elements of predetermined compliance.
It is a further object to provide array members of such flexure span elements arranged in a balanced manner such that all flexure stresses that are lateral to the central stroke axis are made to cancel each other so that automatic lateral compensation ensures that the movement of the mass portion is kept in a precise straight line throughout the stroke length, and that the moving mass portion is constrained against spurious motion such as rotation and rocking.
It is a further object to provide electromagnetic actuators, utilizing laterally compensated flexures in which the linear motor is of the “voice coil” type for uniform driving force throughout the stroke length.
It is a further object to provide an embodiment exhibiting two different mechanical resonant frequencies available for utilization as the driven frequency.
It is a further object to provide an embodiment wherein the mechanical resonant frequency can be conveniently adjusted over a predetermined range.
It is further object to hold the total mass of the inertial actuator to a minimum as required for aircraft and spacecraft.